1. Field of the Invention
This invention relates to a transmission belt with automatic tension adjustment.
2. Brief Description of the Background of the Invention Including Prior Art
A friction power transmission belt as is used for example in connection with pulleys shows in general belt elongation by bending during use, elongation caused by stretching and compressing and also apparent elongation based on loosening due to wear on both sides and falling into the grooves while in use as is the case with a V-belt. When such belt elongation occurs, the belt tension lowers and the belt is no longer capable of transmitting the desired power and the slip of the belt in use increases suddenly. Thus excessive frictional heat is generated and the belt finally breaks down. Conventionally, in order to avoid such problems, the belt is kept in sufficient tension by adjusting the center distance between the two pulleys or by providing a tension maintaining means such as a tension pulley, which generally requires additional space. Another frequently employed method of providing to a belt sufficient tension is to set the initial tension on the pulleys about 30 to 40 percent higher than the tension required for normally transmitting the prescribed power so as to avoid slackening of the belt. However, this method has the disadvantages that the transmission belt reduces its service life rapidly under the excessive tension. At the same time bending of axles and abrasion of bearings are caused by a greater load than necessary applied to axles and bearings.
For example, in the complex construction of recent automobiles, a transmission belt for transmission of power is set into a narrow space. When the transmission belt becomes loose, adjustment of the tension pulleys for tesioning the belt or replacement of the belt become necessary requiring costly labor.
If a conventional transmission V belt is employed in connection with an automobile engine, the belt tension will decrease from the initial tension of the belt due to its strain increasing in proportion to the running distance. Such elongation of the belt can be caused by (1) apparent elongation since the belt falls into the grooves of the pulleys as a result of the advancing abrasion at the two sides of the belt, (2) elongation of the polyester fiber cords as tension providing members of the V belt based on the cords being subjected to stretching and compressing when under almost constant tension and (3) apparent elongation based on the deformation of the cross-sectional shape of the belt caused by lateral pressure applied to the sides of the belt during running. These three factors can bring about a lowering of the belt tension and an elongation of the belt resulting in belt slipping.
Conventionally, when elongation of a belt occurred, the belt tension was adjusted. In order to decrease the amount of work involved in retensioning of belts or to dispense with the work of retensioning belts, Keefe in U.S. Pat. No. 3,469,001 has suggested to use polyester fiber cords having the property that their elongation at 100.degree. C. and that at 24.degree. C. are almost equal with the purpose of minimizing the elongation of the cords. When this belt is stretched onto pulleys a high tension is needed to avoid slipping and/or retensioning while the belt is becoming longer during use. The lowest initial tension to avoid slip of the belt is a tension with 50 percent or more in extra tension. However, with the belt being installed at such excessive tension onto pulleys, a large force is applied to both axles and bearings and as a result the axles can bend and the bearings can wear out. In addition, problems at the initial stage may increase and it may be easy to break the belt by shock. Thus while there is some improvement provided by the belts of U.S. Pat. No. 3,469,001, in particular elongation as a result of the above mentioned factors (1) and (3) remains unimproved and in the course of time belt tension will decrease gradually. Alternatively, if the belt of U.S. Pat. No. 3,469,001 is installed on pulleys at a tension lower than the minimum tension required for power transmission, the belt slips immediately after the start of running the belt and the belt has to be retensioned.
U.S. Pat. No. 3,469,001 employs in example 1 a polyester yarn of a relative viscosity of about 50 and in examples 2 and 5 a polyester yarn of a relative viscosity of 31.5 as measured in a 10 percent solution of the polymer in a mixture of 10 weight parts of phenol and 7 weight parts of 2,4,6-trichlorophenol at 25.degree. C. Relative viscosities of 50 and 31.5, respectively, as determined by the cited method are equivalent to relative viscosities of 0.84 or 0.66 when employing orthochlorophenol as a solvent for determining the viscosity. Polyester cord prepared from polyester with these viscosity properties resulted in a shrinkage tension after thermal stretching of from about 0.31 to 0.39 g/D as measured at 160.degree. C. Also the thermal stretching is performed in one step at a comparatively high elongation percentage of from about 4.5 to 12.3 percent of the belt length.
The polyester fiber cords employed in typical conventional belts are made of polyester fiber yarn having the following characteristics:
Relative viscosity: 0.60 to 0.70 (dissolved in orthochlorophenol) PA1 Tenacity: 7.5 to 8.5 g/D PA1 Elongation: 11 to 15 percent PA1 Heat shrinkage: 5 to 10 percent (held for 30 Minutes at 150.degree. C.) PA1 Shrinkage tension: 0.2 to 0.4 g/D (at 180.degree. to 215.degree. C.) (maximum tension)
In the above properties, g/D represents gram per denier of the fiber. The relative viscosity of the polyester fiber yarn was obtained by dissolving 2 gram of polyester fiber in 25 ml orthochlorophenol; then the viscosity was measured by a Ostwald's viscosimeter; the measured value was divided by the viscosity of orthochlorophenol, multiplied by 0.024 and thereto added 0.02634.
British Pat. No. 1,190,306 to Arntz discloses improvements in V-belt manufacture by vulcanizing a belt under tension caused by a tensile standard load, stretching the belt to bring it to its normal length and cooling the belt to stabilize it at said nominal length.
Waugh in U.S. Pat. No. 4,137,787 employs a cylindrical curing mandrel having a smaller diameter than the diameter of the forming mandrel. Fix in U.S. Pat. No. 3,566,706 discloses that heat-shrinkable elastic or partially elastic cords are simultaneously heat set and relaxed in the absence of applied tension after a rubber to textile bonding agent has been applied.